No OOTB solution for this I think. What you can do is that keep your username and passwords inside the preference properties as illustrated in the link shared by Vijay.
http://soawork.blogspot.co.uk/2012/08/preference-deployment-descriptor-in-soa.html

Retrieve these properties values inside your BPEL and place them in the XML fragment appropriately before assignment.

In this way, your username and password is exposed to Preference property that could be changed through 2 ways:
- From the EM console at Runtime without needing to redeploy the code as stated in the link above
- Through the configuration plan at the time of deployment (which is what you want, as shown below for username)